The invention relates to a disc brake, in particular for a motor vehicle, having a clearance monitoring device. The invention also relates to a method for monitoring the clearance of a disc brake.
Such disc brakes are known in various designs. For their safe and reliable functioning it is necessary that a defined distance, referred to as the clearance, is maintained between their friction partners (brake pads and brake disc), under all operating conditions.
Wear adjustment devices are known in different designs, for example mechanical adjusters which ensure there is constant clearance within certain limits in the pneumatically acting disc brakes which are customary nowadays in heavy commercial vehicles. In this context, whenever the brakes are activated the adjustment device is actuated, for example, by means of a positioning element of a brake application device of the disc brake. In the case of wear of brake pads and the brake disc, automatic adjustment of the pads takes place by means of the adjustment device, for example by means of an adjustment movement of pressure rams with variable length.
A structural predefined clearance is formed as fixed geometric variables in the components which are involved in the adjustment process. Continuous monitoring of the clearance is not possible owing to the purely mechanical conditions or because of a lack of sensors. Therefore, nowadays only manual measurement of the clearance, to be performed within the scope of the regular brake inspection, is provided. However, this checking takes place, of course, only at relatively long time intervals or mileage or kilometerage intervals, for example when changing brake pads and, furthermore, only in the cold state of the disc brake. Therefore, the clearance remains unobserved over long driving distances and during different operating states of the disc brake, and any critical changes remain undetected.
Disc brake designs are known which permit the clearance to be monitored and, moreover, even permit it to be set actively or corrected during operation. For example, this is carried out in a pneumatically activated disc brake in that the adjustment device is driven electrically, and therefore the brake pads are moved relative to the brake disc. A corresponding control logic uses operating parameters of the electric drive to measure the clearance which is present and, if appropriate, set a desired clearance. DE 19731696 A1 illustrates an example of this.
However, such designs have previously not been commercially successful due to the considerable technical development leap as well as the associated costs.
A further type of brake which can permit monitoring of clearance is known in the form of an electromechanical disc brake. The brake mechanism is activated here purely electrically, and monitoring and setting of the clearance can be carried out with the control electronics which are necessary for braking However, this design constitutes an even greater technological step, and the use of such systems in commercial vehicles on a standard basis can not be anticipated. An example of this is illustrated by document DE 19933962 C2.
Therefore, the object of the present invention is to improve a disc brake, in particular a pneumatic disc brake, of a conventional design with clearance monitoring and with the smallest possible structural expenditure and expense.
A further object is to provide an improved method for monitoring clearance.
These and other objects are achieved by way of a disc brake, and corresponding method, in accordance with embodiments of the invention.
A basic concept of the invention is that a clearance monitoring device is provided with a control device which is connected to the wear sensor and to the brake control unit. An already existing wear sensor, which is frequently used in disc brakes, is therefore used.
A disc brake according to the invention, in particular for a motor vehicle, comprises a brake-application device, in particular one having a brake lever, an adjustment device which is coupled to the brake application device, in particular to the brake lever, in order to adjust the wear of brake pads and of a brake disc, a wear sensor for detecting a wear value of the brake pads and the brake disc, and a brake control unit. The disc brake has a clearance monitoring device with a control device which is connected to the wear sensor and to the brake control unit.
This provides the advantage that a disc brake with a clearance monitoring device is provided with the smallest possible expenditure.
A method according to the invention for monitoring clearance of such a disc brake comprises the acts of: forming current value pairs from current brake pressure values and detected current signal values of the wear sensor during a braking process; comparing the formed current value pairs with previously stored reference values; and evaluating the comparison and outputting of messages in order to monitor the clearance.
It is therefore advantageously possible to detect and display or report unacceptable deviations from defined or previously specified clearance values.
In one embodiment, the control device of the clearance monitoring device is designed to detect a current signal value of the wear sensor with a detection unit. The detection unit can detect a change in the current signal value of the wear sensor over time. This is particularly advantageous when the wear sensor is adjusted. A change can therefore be detected immediately.
A further embodiment provides that the control device of the clearance monitoring device is provided for forming current value pairs from current brake pressure values and detected current signal values of the wear sensor and for comparing the current value pairs with stored reference values with a comparator unit. The reference values can be, for example, pressure travel characteristic curves of a brake cylinder of the associated disc brake. It is particularly advantageous if these pressure travel characteristic curves can be learned in the new state, which permits a different embodiment.
The current brake pressure values can therefore originate from the brake control unit and/or can be output values of at least one other sensor. The brake control unit can have corresponding table values and/or characteristic curve values. Of course, an existing pressure sensor or else force sensor can also be used as well. The stored reference values are also stored in the brake control unit.
Furthermore, the stored reference values can be stored in a memory unit of the control device. It is advantageous here if these are the learned characteristic curve values and further learned values, since greater precision is then possible.
In yet a further embodiment, the control device of the clearance monitoring device has an evaluation unit for evaluating the results of the comparator unit. In this way, the detected values can be differentiated on the basis of trends and secondary conditions. Warnings and displays can be effected precisely in this respect.
In addition, there is provision that the clearance monitoring device comprises a signaling unit which signals messages about the state of the clearance monitoring acoustically, visually, haptically and/or alphanumerically on the basis of the evaluation of the evaluation unit. A driver of the vehicle can therefore be alerted early enough to problems or compliance with maintenance periods or the need to look for a workshop. Furthermore, the messages can be stored and can be retrievable by maintenance personnel.
In order to include secondary conditions and to estimate trends and detected measured values, the clearance monitoring device can also have at least one temperature detector. In this context, already existing temperature sensors or else additional ones can be used.
The control device of the clearance monitoring device can be a component of the brake control unit, as a result of which no significant additional space requirement results for the disc brake.
In the method step of forming current value pairs (p/V), the current brake pressure values are supplied by the brake control unit and/or an additional sensor. The brake pressure values are already present in the brake control unit, and therefore no additional sensors requiring space have to be installed.
In one embodiment there is provision that in the method step of evaluating, a previously definable nominal clearance without adjustment is detected if a current value pair corresponds to a stored value pair which is assigned to a correct clearance, and the following value pair does not have any change in the detected current signal value of the wear sensor but does have a strong rise in the brake pressure. In this way, simple differentiation of the further cases is possible.
In this context, in the method step of evaluating, previously definable nominal clearance, enlarged owing to wear, with adjustment is detected if a current value pair corresponds to a stored value pair which is assigned to a correct clearance, and the following value pair has a change in the detected current signal value of the wear sensor but does not have a strong rise in the brake pressure.
And for a further case, in the method step of evaluating, a previously definable nominal clearance is detected as being undershot if the brake pressure of a current value pair is lower than the brake pressure which is assigned to a previously definable, correct clearance, and the subsequent value pair does not have any change in the detected current signal value of the wear sensor but does have a strong rise in the brake pressure or has a change in the detected current signal value of the wear sensor but does not have a strong rise in the brake pressure.
In this way, differentiation between cases can advantageously be performed on the basis of the value pairs, in order to monitor the clearance.
Furthermore, in the method step of evaluating, thermal influences are also included by means of a temperature detector, as a result of which the reliability of the messages is increased.
A further increase in the reliability is achieved in that in the method step of evaluating, a warning or a warning signal is not output until after a specific number of braking operations.
An increase in the reliability is also possible with previously carried out learning of the reference values in the brake control unit and/or the memory unit.
Furthermore, in a further embodiment it is advantageously permitted that a friction point can be detected in that the current value pairs are compared with stored value pairs if a current value pair of the current value pairs which are compared with stored value pairs has a strong rise in the brake pressure.
With the clearance monitoring device according to the invention and the corresponding method according to the invention for monitoring clearance, the current clearance can be detected and monitored continuously or sufficiently frequently.
Differentiated detection of enlarged correct and undershot clearance is possible.
The clearance can be monitored and detected specifically for each brake.
The expenditure for this is minimized in that no additional space is necessary and output values of existing functional units continue to be used.
Additional sensors on the wheel brake for detecting the clearance or clearance gap are not necessary. The associated separate electronic evaluation units (setpoint/actual value comparison) and output units can be dispensed with.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.